Short Answer
Your birthday is enough to find your Sun sign. Your main zodiac sign is the sign range where the Sun was on your date of birth. For a more personal reading, the Moon sign, rising sign and houses in the birth chart should also be considered.
Choose your birthday and instantly see your Sun sign, element and nearby themes.
In short
Finding your sign is usually simple: your birth date gives your Sun sign. But if you were born on a cusp — the day a sign changes — the date alone is not enough, because the exact hour the Sun changed sign matters.
The Sun completes the zodiac in one year and spends about a month in each sign. Whichever sign the Sun occupied on your birth day is your Sun sign, which is why the date alone is enough for most people.
The Sun sign describes your conscious identity and life direction. It is the usual answer to "what is your sign?", yet it is only one part of your birth chart.
If you were born on a day when signs change — often around the 20th to 23rd — looking at the date alone can mislead you. The moment the Sun changes sign shifts by a few hours from year to year.
If you were born on such a day, your birth time is needed to settle the question. Otherwise it stays unclear which of two neighbouring signs is yours, and the whole reading rests on the wrong basis.
The Sun sign is a good start but incomplete on its own. Your Moon sign describes emotional needs, and your rising sign your outward manner and the arrangement of houses in your chart.
Together these are called the big three, and they explain why "my sign does not fit me" is such a common feeling. Moon and rising also require your birth time and place.
FAQ
Usually not — the date is enough. But if you were born on a cusp day when the sign changes, the time is needed because the Sun may have changed sign during that day.
This is called a cusp. A calculation with your birth time gives the definite answer; in astrology you are born into one sign, not a blend of two.
Look at your Moon and rising signs. All three together describe you far more accurately.